Hello!
If I disable Windows Ink, I lose pen pressure. And if I leave it on, I’m stuck with a very annoying lag every time I try to move a slider.
Any idea what I should do? :o
Thanks a lot! 
It might be helpful to know the hardware involved, but as far as general tablet issues:
Always be using the most up to date version of Zbrush. Obsolete versions may be missing fixes, and are not officially supported.
Zbrush uses the Wintab API for pressure sensitivity. Make sure your tablet is compatible, and has the wintab drivers installed. You may have to install these separately if your tablet model doesn’t use these by default.
The Windows OS has a number of features enabled by default that are garbage for performance tablet users. Find your OS pen and tablet settings, and disable any sort of press and hold features, visual effects, or any sort of gesture features like Pen Flicks that make the OS evaluate the behavior of every stroke. These can cause momentary input lag.
If using a wacom tablet, it can sometimes be helpful to decrease or disable the double click distance in the Wacom tablet Properties.
Otherwise you will need to contact Support, and provide them with the information they will need to assist you.
Thanks!
I’ll try that!